Particulate emissions and respiratory exposure to hazardous chemical substances during additive manufacturing of sand moulds
Background: AM (Additive Manufacturing) is a process that produces three-dimensional (3D) parts via the layering of materials. To date, there has been little research on the health risks associated with the use of this technology to produce sand moulds for metal casting. Apart from the processing steps involving the AM machine, binder jetting (BJ) of sand moulds includes an additional step which involves the coating of virgin (new) silica sand with sulphonic acid prior to the AM processes. As yet, there is also no information available on the risks concerning the coating process. Both coating and AM processes involve the handling of silica sand, which may result in personal exposure to respirable crystalline silica and particulate matter. Also, furan resin binders used during processing may cause emission of volatile organic compounds (VOCs). Objectives: This study was aimed at determining emissions of and personal respiratory exposure to hazardous chemical substances (respirable crystalline silica, VOCs and respirable particulate matter) during various phases of AM with silica sand as well as during the coating of silica sand prior to AM. Methodology: Analysis of bulk silica sand was performed through the characterisation of particle size and shape using Malvern Morphologi particle analyser and scanning electron microscopy (SEM) while the X-ray diffraction (XRD) process was utilised to determine elemental composition. The Aerosol Particle Counter (APC) and Condensation Particle Counter (CPC) were used to quantify emissions of 0.3 - 10 μm (APC) and 0.01 - ~ 1.0 μm (CPC) sized particles during the coating process as well as pre-processing, processing and post-processing phases. Both personal and area monitoring were performed to measure airborne concentrations of respirable crystalline silica, VOCs and respirable particulate matter during the operator’s shift. Three days were allocated to monitor emissions, personal exposure and area concentrations during coating, while monitoring of the AM process was conducted over a period of five days, where three identical parts were printed. Results: Respirable sized particles were found in all three sand types (virgin, coated and used), with virgin sand having the highest respirable content compared to other sands; d(0.9) = 3.98 ± 0.72 μm vs d(0.9) = 115.00 ± 95.15 μm and d(0.9) = 6.51 ± 2.71 μm, respectively. The quartz content for the three sand types ranged from 92.6 to 97.6%, which made it the dominant mineral. The coating and AM machines were confirmed to emit particles 0.3 μm, 0.5 μm, 1 μm and 0.01 - ~ 1.0 μm. in size. Particulate emissions data indicated that the particle emissions for particles 0.3 μm, 0.5 μm and 1 μm and 0.01 - ~ 1.0 μm in size, increased considerably during the cleaning of the coating machine filter as well as during the opening of the bay door. The particle number concentrations of 0.3 μm, 0.5 μm, 1 μm and 0.01 - ~ 1.0 μm sized particles were higher during the third day of printing compared to the other days, as a result of increased particle number concentrations during post-processing and pre-processing. When comparing the phases, the average particle number concentration was significantly higher during pre-processing compared to other phases. There were no significant differences in emission rates (ERs) but the average ERs for particles 0.3 μm and 0.01 - ~ 1.0 μm in size were slightly higher during pre-processing followed by post-processing and then processing. Personal exposure to respirable crystalline silica during cleaning of the coating machine with compressed air exceeded the South African time weighted average occupational exposure limit-control limit (TWA OEL-CL) of 0.1 mg/m3 (0.112 mg/m³). During cleaning of the AM machine, personal exposure to respirable crystalline silica surpassed the action level (0.07 mg/m3). The personal exposures to HCS (respirable crystalline silica, respirable dust and VOCs) measured during the AM processes were below 10% of their respective OELs. Conclusion: The feedstock material used (silica sand) was regarded as a primary source of exposure. The amount of respirable sized particles found in the silica sand coupled with quartz content of > 90%, made the feedstock material a risk to the health of the AM operator. Both the cleaning methods used during coating and printing led to increased personal exposure to respirable crystalline silica. The AM operator was overexposed to respirable crystalline due to the use of compressed air to clean the filter, and was exposed to respirable crystalline silica exceeding the action level due to the use of a broom when cleaning the AM machine room. The cleaning methods were therefore classified as tasks with the highest risks. Particulate emissions were higher during pre-processing compared to other processes and is of concern since emissions of particles 0.3 μm, 0.5 μm, 1 μm and 0.01 - ~ 1.0 μm in size are a risk to the health of the AM operator as they may deposit in the alveoli, resulting in respiratory dysfunctions. In an attempt to minimise exposure to HCSs during the AM of sand moulds, 18 recommendations were made to the AM facility. Due to overexposure caused by cleaning activities using compressed air, the key recommendation made was for the AM operator to clean the filters in an isolated glove box in order to reduce exposure to respirable crystalline silica, a confirmed human carcinogen.
- Health Sciences